1. Field of the Invention
This invention relates to photoreceptors suitable for application to xerographic printers and like machines that use coherent light sources and methods for fabricating photoreceptors. More particularly, this invention relates to a multilayered photoreceptor having a conductive substrate having a designated surface roughness that eliminates an interference-fringe print defect in the resulting printer output and enables the use of undercoat layer materials, such as an organometallic or organometallic chelate compound with a silane, examples of which include any suitable hydrolyzable organozirconium, organotitanium or organoaluminum compound with a silane. The invention also relates to a fabrication method for forming a substrate, for example a metallic substrate, of a multilayered photoreceptor to produce a specific surface morphology and surface roughness, and coating the roughened substrate with the undercoat film without a thickening agent.
2. Description of Related Art
Xerographic printers and like machines that use multilayered photoreceptors in conjunction with a coherent light source suffer from an interference effect that manifests as a printable defect that can be described as a series of dark and light interference fringes that resemble wood grains. The use of coherent illumination sources in conjunction with multilayered photoreceptors produces the interference effect through the interaction between various reflected components of the incident light whose difference in optical path length varies from one area of the photoreceptor to another. Such spatial variation in the optical path length arises because the coated layers have inherent spatial thickness variations imposed by limitations in the coating process. The spatial variation in the optical path length in turn produces absorption variation in the charge generating layer of the photoreceptor, resulting in the interference-fringe defect in prints generated by these xerographic machines.
FIG. 1 is a schematic view of a typical photoreceptor of a multilayered design. In FIG. 1, the photoreceptor 10 includes a substrate 1, an undercoat layer 2, a charge generating layer 3, and a charge transport layer 4.
In the present device, which comprises three organic layers 2-4 coated on a metallic substrate 1, an incident light beam 5 is directed at the charge transport layer 4. The primary light beam 5 is then reflected from the planes that define interfaces 7, 9A, 9B and 9C between the layers 1-4 of the multilayered photoreceptor. More specifically, reflected light beam 6 is generated via reflection from the interface 7 between the atmospheric air and the charge transport layer 4, reflected light beam 8A is generated via reflection from the interface 9A between the charge transport layer 4 and the charge generating layer 3, reflected light beam 8B is generated via reflection from the interface 9B between the charge generating layer 3 and the undercoat layer 2, and reflected light beam 8C is generated via reflection from the interface 9C between the undercoat layer 2 and the substrate 1. The primary reflections that contribute to the interference-fringe print defect producing interference effect are the reflected beam 6 generated at the interface 7 between the surrounding atmospheric air and the charge transport layer 4 and the reflected beam 8C from the interface 9C between the undercoat layer 2 and the substrate 1, where the differences in optical indices are the greatest.
Many methods have been proposed to suppress the charge transport layer/air interface specular reflection, including roughening of the charge transport layer surface by introducing SiO.sub.2 and other particles into the charge transport layer, applying an appropriate overcoating layer and the like.
Many methods have also been proposed to suppress the intensity of substrate surface specular reflection, e.g., coating specific materials such as anti-reflection materials and light scattering materials on the substrate surface and roughening methods such as anodization, dry blasting and liquid honing of the substrate surface. However, such methods must achieve their primary objective of eliminating substrate surface reflections without adversely impacting the electrical parameters or print quality of photoreceptors into which they are incorporated.
Patents on interference-fringe effect suppression in general and suppression of the substrate surface reflection in particular include Tanaka et al. U.S. Pat. No. 4,618,552 (adding an opaque conductive layer above the ground plane), Nagy de Nagybaczon et al. U.S. Pat. No. 4,741,918 (coating process using a buffing wheel), Kubo et al. U.S. Pat. No. 4,904,557 (roughened photosensitive layer on top of a smooth substrate surface), Fujimura et al. U.S. Pat. No. 4,134,763 (grinding method to roughen the substrate surface), Simpson et al. U.S. Pat. No. 5,096,792 (addition of antireflection layer on top of the substrate surface), and Andrews et al. U.S. Pat. No. 5,051,328 (Indium Tin Oxide transparent ground plane as the substrate).
A liquid honing process, for example, is an effective technique to create a highly scattered surface on a metallic substrate, and is used in some multilayered devices to eliminate the interference-fringe effect. The method, however, has several disadvantages which the present invention overcomes.
For example, the liquid honing process is an added step following diamond lathing which thereby increases the cost of production of a substrate. The surface morphology of the substrate created by liquid honing method also does not lend itself to be used in conjunction with a thin-film forming undercoat layer material, such as the aforementioned organometallic or organometallic chelate compound with a silane, due to the nature of the surface texture that is undesirable for a thin-layer coating in the thickness range of approximately 0.05-0.5 .mu.m and still provides complete surface coverage of the substrate. It is required that the thin-film forming undercoat layer materials provide continuous coverage of the underlying metallic substrate in order that print defects due to charge injection from the substrate are eliminated.
In typical multilayered photoreceptors, a resins layer is inserted as an undercoat layer between the substrate and the photosensitive layers in order to provide mechanical strength, better adhesion between the substrate and the photosensitive layers and improved cyclic stability. Each intermediate layer may be any layer conventionally employed between the substrate and the photosensitive layer as illustrated for example in Tanaka et al., U.S. Pat. No. 4,618,552 and Andrews et al., U.S. Pat. No. 5,051,328, the disclosures of which are incorporated herein by reference. Accordingly, the intermediate layer may be a subbing layer, barrier layer, adhesive layer, and the like. The intermediate layer may be formed of, for example, casein, polyvinyl alcohol, nitrocellulose, ethyleneacrylic acid copolymer, polyamide (nylon 6, nylon 66, nylon 610, copolymerized nylon, alkoxymethylated nylon, and the like), polyurethane, gelatin, and the like. Intermediate adhesive layers between the substrate and the subsequently applied layers may be desirable to improve adhesion. Typical adhesive layers include film-forming polymers such as polyester, polyvinylbutyral, polyvinylpyrrolidone, polycarbonate, polyurethane, polymethyl methacrylate, and the like as well as mixtures thereof. The intermediate layer may be deposited by any conventional means such as dip coating and vapor deposition and preferably has a thickness of from about 0.05 to about 5 microns.
Typical resin layers, however, exhibit poor environmental cyclic stability due to the fact that the volume resistivity of a resin greatly depends on the ionic conductivity and is strongly affected by temperature and humidity conditions. Many proposals have been made to form an undercoat layer using organic metal compounds or silane coupling agents to improve upon the environmental effects. Okano et al. U.S. Pat. No. 5,252,422 and Hodumi et al. U.S. Pat. No. 5,188,916, for example, discuss the use of organic metal chelate compounds or organic metal alkoxide compounds with silane coupling agents as an improved undercoat layer in a multilayered photoreceptor for visible light xerographic applications. When this type of an undercoat material is used in combination with a roughened substrate for interference fringe suppression for printer applications where a coherent exposure light source is used, an addition of a resin is required to increase the thickness of the undercoat layer to ensure continuous coverage to avoid charge injection from the substrate. Examples of a print defect caused by charge injection from the substrate include a cluster of black spots in a white background in a discharge area development (DAD) system, which are commonly known as "pepper spots."
Thick undercoat layers, however, produce undesirable electrical effects including a high residual voltage build up and poor cyclic stability. An example of a thick undercoat layer including an organometallic or an organometallic chelate compound is a mixture of acetylacetone zirconium tributoxide and .gamma.-aminopropyltrimethoxysilane and solvents and a polyvinyl butyral resin added as a thickening agent. In order to suppress residual build up in a low temperature and low humidity condition in particular, fabrication using this type of a mixture containing a resin requires a humidification step that results in increased unit manufacturing costs and decreased throughput efficiency. Hongo et al. U.S. Pat. No. 5,286,591 also discloses a subbing layer containing an organic chelate compound or an organic alkoxide compound but with a hygroscopic compound having at least two carboxyl groups per molecule to improve upon environmental cyclic stability. A resin binder is also used to increase the thickness of the subbing layer in the case where interference-fringe image defect suppression is required via roughening of a substrate using a liquid honing method.